The present invention relates to optical amplifiers having operating wavelengths longer than main emission peak wavelengths, and more particularly to erbium doped fiber and waveguide amplifiers operating in the long wavelength regime (1560-1620 nm), especially for wavelength division multiplexing (WDM) applications.
Conventional erbium doped fiber amplifiers (EDFA) have been extensively used in optical telecommunications as means to amplify weak optical signals in the third telecommunication window (near 1550 nm) between telecommunication links. Much work has been done on the design of these amplifiers to provide efficient performance, such as high optical gain and low noise figure. However, with the recent enormous growth of data traffic in telecommunications, owing to the Internet, intranets, and e-commerce, new optical transmission bandwidths are required to provide increased transmission capacity in dense wavelength division multiplexing (DWDM) systems.
There are a few solutions to this demand. One proposed solution is to utilize new materials compositions as a host for the fiber gain medium (instead of silica) such as telluride, which may provide broader amplification bandwidth (up to 80 nm). However, the non-uniform gain shape and poor mechanical properties of telluride glass make these amplifiers difficult to implement in the telecom systems. Also, Raman amplifiers can be considered as an alternative solution to high bandwidth demand, since these amplifiers are capable of providing flexible amplification wavelength with a broad bandwidth. However, these amplifiers place restrictions on optical system architectures because of their required designs for efficient performance, such as long fiber length ( greater than 5 km), high pump power ( greater than 500 mW) and co-pumping configurations. On the other hand, relatively long erbium doped fibers (EDFs) may also provide amplification in the long wavelength range (1565-1625 nm) when they are used with high power pump sources. This range is commonly called xe2x80x9cL bandxe2x80x9d. The conventional range, also known as xe2x80x9cC bandxe2x80x9d is in the wavelength range between 1525-1565 nm.
In principle, L band amplifiers take advantage of the fact that EDFs have a higher e-emission cross-section than absorption cross-section at longer wavelengths. Therefore, for long EDFs, amplified spontaneous emission (ASE) becomes more emphasized at long wavelengths. However, there are still several issues for optimization of L band amplifiers for efficient performance. So far, reported performance of L band EDFAs has been inferior to that of C band EDFAs, with drawbacks as evidenced by higher noise figure (NF) and lower output power and gain. It would be beneficial to provide an L band amplifier with a low noise figure and high output power and gain.
Briefly, the present invention provides an L band optical amplifier. The optical amplifier comprises a signal line for transmitting a signal light in a first direction. The signal line includes an input, an output disposed optically downstream of the input and an amplifying gain medium optically disposed between the input and the output. The optical amplifier further comprises a laser optically connected to the first amplifying gain medium and means for directing C band light generated in a second direction, opposite the first direction, into the amplifying gain medium.
Additionally, the present invention provides a method of amplifying a light signal. The method comprises providing an L band amplifier having a signal line including an input, an output disposed optically downstream of the input and an amplifying gain medium optically disposed between the input and the output. The optical amplifier further includes a laser optically connected to the amplifying gain medium and means for directing C band light into the signal line.
The method further provides providing a light signal into the amplifier input; directing the light signal through the amplifying gain medium; generating a pump signal from the laser into the signal line toward the amplifying gain medium, the pump signal combining with the light signal to produce first amplified light in the amplifying gain medium; generating C band light in the amplifying gain medium, the C band light being directed toward the input; using the means to direct the C band light into the signal line; using the C band light to amplify the light signal in the amplifying gain medium to produce second amplified light; and directing the first and second amplified light toward the output.